Error: Data structure returned is not convertible in the requested format. Try to call this method with the parameters '&format=original&serialize=1'; you will get the original php data structure serialized. The data structure looks like this: $data = array ( 0 => array ( 'login' => 'anonymous', 'alias' => 'anonymous', 'role' => 'noaccess', 'capabilities' => array ( ), ), );